Why do I keep getting these bad thoughts and fear that my girlfriend is going to cheat on me or leave me? I want to know why. I pray and they go away, then they come back again. I want to know why. Do I need help or is there something wrong? I've asked God for forgiveness. I don't know if this is a test that He is putting upon me, but I sometimes feel hopeless without the Lord. My girlfriend and I really love each other, and we have the same values in Christ. We both accepted God into our lives and as our personal Savior. Also, we believe in no sex before marriage. We trust each other. She and I have been together for four months already. That's the longest I've ever been with someone. It's the same for her. We care about each other. We are afraid to lose each other. We're both scared for our future. Someday we want to get married and have kids. I just want to know why I have this fear and bad thoughts all the time. Please help.
You answered your own question, though you didn't realize it. In all your past relationships, you didn't stay with someone as long as you have now stayed with her. You are now in uncharted waters, and you are convinced that this good thing just has to end because of your past experience. Four months is really not that long of a period of time. I don't know what the future will hold for you two, but I do know that life tends to go in the direction that you focus toward. "The lamp of the body is the eye. Therefore, when your eye is good, your whole body also is full of light. But when your eye is bad, your body also is full of darkness" (Luke 11:34). If you are convinced that she is going to leave, you are going to end up driving her away with your fears.
Right now, love is still developing. I know it feels like it is already there, but that is more due to your inexperience with true love. I know it isn't fully there yet because you haven't reached the "bears all things, believes all things, hopes all things, endures all things" (I Corinthians 13:7) stage yet. Don't try to rush it. Love is one of those things that if you try to force it to happen, you lose it. "I charge you, O daughters of Jerusalem, By the gazelles or by the does of the field, Do not stir up nor awaken love Until it pleases" (Song of Solomon 2:7).
She is not like your past girlfriends. You are not like her past boyfriends. Whether this relationship will grow into a love that leads to marriage is still to be seen, but whether it does or not depends on who you two are, not what happened in the past.
In the same way, you and your girlfriend are on the way to becoming Christians, but you haven't reached the destination yet. For more information see What Must I Do to be Saved?
Thanks. This helps a lot